This invention relates to reducing the sulfur content of a sulfur-containing hydrocarbon stream. In particular, the invention relates to a method which removes sulfur from the hydrocarbon stream under mild conditions.
Heavy petroleum fractions, such as vacuum gas oil or resides may be catalytically cracked to lighter and more valuable products. The product of catalytic cracking is conventionally recovered and the products fractionated into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions, such as heating oil and diesel fuel; lube fractions; and heavier fractions.
Generally, sulfur occurs in petroleum and petroleum products as hydrogen sulfide, organic sulfides, organic disulfides, mercaptans, also known as thiols, and aromatic ring compounds such as thiophene, benzothiophene (BT), dibenzothiophene (DBT) and their alkylated homologues. The sulfur in aromatic sulfur-containing ring compounds will be herein referred to as xe2x80x9cthiophenic sulfurxe2x80x9d.
Where a petroleum fraction is being catalytically cracked and contains sulfur, the products of catalytic cracking usually contain sulfur impurities which normally require removal, usually by hydrotreating, in order to comply with the relevant product specifications. Such hydrotreating can be done either before or after catalytic cracking.
Conventionally, feeds with substantial amounts of sulfur, for example, those with more than 500 ppm sulfur, are hydrotreated with conventional hydrotreating catalysts under conventional conditions, thereby changing the form of most of the sulfur in the feed to hydrogen sulfide. The hydrogen sulfide is then removed by amine absorption, stripping or related techniques. Unfortunately, these techniques often leave some traces of sulfur in the feed, including thiophenic sulfur, which are the most difficult types to convert.
The ease of sulfur removal from petroleum and its products is dependent upon the type of sulfur-containing compound. Mercaptans are relatively easy to remove, whereas aromatic compounds such as thiophenes are more difficult to remove. Of the thiophenic sulfur compounds, the alkyl substituted dibenzothiophenes are particularly resistant to hydrodesulfurization.
The sulfur impurities in petroleum fractions which boil in either the distillate boiling range, such as diesel fuel, or the gasoline range are usually removed by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations both of which are expected to become more stringent in the future, possibly permitting no more than about 30-50 ppmw sulfur in both diesel fuel and motor fuel gasolines. Low sulfur levels can contribute to reduced emissions of CO, NOx and hydrocarbons.
Hydrotreating any of the sulfur containing fractions which boil in the distillate boiling range, such as diesel fuel, causes a reduction in the aromatic content and, therefore, an increase in the cetane number of diesel fuel. While hydrotreating reacts hydrogen with the sulfur containing molecules in order to convert the sulfur and remove it as hydrogen sulfide, as with any operation which reacts hydrogen with a petroleum fraction, the hydrogen does not only react with the sulfur as desired. For example, other contaminant molecules containing nitrogen undergo hydro-denitrogenation in a manner analogous to hydrodesulfurization. Unfortunately, some of the hydrogen may also cause hydrocracking, as well as aromatic saturation, especially during more severe operating conditions of increased temperature and/or pressure. Typically, as the degree of desulfurization increases, the cetane number of the diesel fuel increases; however this increase is generally slight, usually from 1-3 numbers.
Hydrotreating can be effective in reducing the level of sulfur to moderate levels, e.g. 500 ppm, without a severe degradation of the desired product. However, to achieve the levels of desulfurization that will be required by the new regulations, almost all sulfur compounds will need to be removed, even those that are difficult to remove such as DBTs. These refractory sulfur compounds can be removed by distillation, but with substantial economic penalty, i.e., downgrading a portion of automotive diesel oil to heavy fuel oil.
Cracked naphtha, as it comes from a catalytic or thermal conversion process and without any further treatments, such as purifying operations, has a relatively high octane number, due, in part, to the presence of olefinic components. As such, cracked naphtha is an excellent contributor to the gasoline pool, providing a large quantity of product at a high blending octane number. In some cases, this fraction may contribute as much as up to half the gasoline in the refinery pool. In special situations, where a refinery has no catalytic reformer, the cracked naphtha may represent as much as 80% of the refinery""s gasoline.
Hydrotreating of any of the sulfur-containing fractions of cracked gasoline causes a reduction in the olefin content. Current sulfur specifications can often be met without excessive octane loss by hydrotreating only the heaviest, most sulfur-rich and olefin-poor portion of the FCC gasoline. As the future pool sulfur specification is reduced, increasing amounts of lighter boiling, olefin-rich, gasoline must be processed and the octane penalty can increase dramatically due to olefin saturation in these lighter gasoline fractions. The decrease in octane which takes place as a consequence of sulfur removal by hydrotreating creates a tension between the need to produce gasoline fuels with sufficiently high octane number and, because of current ecological considerations, the need to produce cleaner burning, less polluting fuels, especially low sulfur fuels.
Methods have been proposed for offsetting pool octane reductions which could occur if severely hydrotreated, wide-cut FCC gasoline were introduced into the pool. Catalytic reforming increases the octane of virgin and hydrocracked naphthas by converting at least a portion of the paraffins and cycloparaffins to aromatics in these very low olefin content feeds. Reforming severity might be boosted to further increase the octane of the reformate going into the gasoline pool, thereby offsetting the negative impact on the pool from blending hydrotreated wide cut FCC gasoline. This approach, however, has two limitations. First, reformate yield declines as severity is increased which could negatively impact the total gasoline pool volume. Second, as already noted, aromatization reactions account, to a large degree, for the octane enhancement in reforming. Aromatics, however, particularly benzene, have been the subject of severe limitations as a gasoline component because of possible adverse effects on the ecology. It has therefore become desirable, as far as is feasible, to create a gasoline pool in which the higher octanes are contributed by non-aromatic components.
As noted above, the more restrictive gasoline pool sulfur specifications that are anticipated often will not be met by processing only the heaviest, sulfur-rich olefin-poor portion of the FCC gasoline. For this reason, the lighter components, including light and possibly full range FCC naphthas, will have to be treated to achieve acceptable sulfur levels. However, the octane loss, associated with hydroprocessing, or yield loss, associated with processes aimed at recovering that lost octane, can increase dramatically as the boiling point range of the gasoline feed being treated widens.
Consequently, it is desirable to develop methods for preserving yield and octane while removing sulfur from the relatively olefin-rich light and mid-range portions of the FCC gasoline pool.
Thus, there remains a need for a method of removing sulfur from hydrocarbon feeds which contain sulfur compounds, including thiophenic sulfur compounds, under moderate process conditions and maintaining the characteristics of the feed stream.
The present invention is directed to a method for reducing the sulfur content of a sulfur-containing hydrocarbon stream under mild conditions. A process is provided in which sulfur is removed from a sulfur-containing hydrocarbon liquid feed stream by:
contacting the hydrocarbon stream with transition metal particles, containing the transition metal in a zero oxidation state and having an average diameter of less than about 200 nm, under reaction conditions sufficient to provide a product having a reduced sulfur content and metal sulfide particles; and
separating the metal sulfide particles from the product.
In a preferred embodiment, the transition metal particles are provided by adding a source of transition metal precursors to the liquid feed stream and sonicating the liquid feed stream/metal precursor combination under conditions sufficient to produce the transition metal particles.
Preferably, the source of transition metal precursors includes a transition metal carbonyl precursor.
The transition metal carbonyl precursor is preferably of the formula:
Mn(CO)x 
wherein M is a transition metal selected from an element from Groups 6 and 8-12 of the period table, n is an integer from 1 to 6 and x is an integer from 4 to 16.
Preferably, the transition metal is selected from Fe or Mo, with the corresponding transition metal precursor being Fe (CO)5 and Mo (CO)6, respectively.
The source of transition metal precursors is preferably added to the liquid feed stream in an amount sufficient to provide at least a 1:1 molar ratio of metal: sulfur.
The sonication conditions can include contacting the feed stream with sonic energy having a frequency in the range of about 1 Hz to about 20 kHz, at a temperature in the range of about 10 to about 150xc2x0 C. and a sonication residence time in the range of about 1 second to about 2 hours.
The separating step can be accomplished through at least one of settling out, decanting, filtration or centrifugal separation.
In a preferred embodiment, the present invention is directed to a method for reducing the sulfur content of a sulfur-containing hydrocarbon liquid feed stream under mild conditions, in which the method includes:
adding a transition metal carbonyl precursor to the liquid feed stream;
sonicating the liquid feed stream containing the transition metal carbonyl precursor at a temperature in the range of about 10xc2x0 C. to about 150xc2x0 C. for a time sufficient to produce solid metal sulfide particles and a liquid product having a reduced sulfur content; and
separating the solid metal sulfide particles from the product.
The method can also include adding a solvent to the liquid feedstream prior to the sonicating step.
The present invention provides a method for reducing the sulfur content, including thiophenic sulfur compounds, of a sulfur-containing hydrocarbon stream under mild conditions. The resulting product stream will have a reduced sulfur content, while preserving the yield, chemical composition and motor fuel performance characteristics, e.g., octane, of the feed stream.
Additional objects, advantages and novel features of the invention will be set forth in part in the description and examples which follow, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.